The STEN name came out of names of the designers (R. V. Shepard and H. J. Turpin) and from the factory where they worked (Enfield arsenal). It was one of the most crude and ugly and simply, but effective submachine guns of the WW2. Almost 4 millions of STEN guns of different versions were made between 1941 and 1945. STEN guns were made not only in Royal Small Arms factory in Enfield; other makers included famous British gunmaking company of the time BSA Ltd, as well as Royal Ordnance Arsenal in Fazakerly, England, and Long Branch Arsenal in Canada.

The first STEN, STEN Mk.I (full official name was 9mm STEN Machine Carbine, Mark 1), was developed in mid-1941. It was blowback operated, automatic weapon that fired from the open bolt. Trigger unit permitted for sigle shots and full automatic fire, controlled by the cross-bolt type button, located in front and above trigger. The tubular receiver and the barrel shroud were made from rolled steel. The gun was fed from left side mounted box magazines. The stock was of skeleton type, made from steel. Sights were fixed, pre-adjusted for 100 yards distance, peep hole rear and blade front. The Mk.1 featured spoon-like muzzle compensator. Some guns featured small folding forward grip. Total production of Mark 1 and slightly modified Mark 1* STEN machine guns was about 100 000.

The STEN Mk.II submachine gun was most widely made gun in entire STEN series, with about 2 millions of Mark 2 being made during the war. It was slightly smaller and lighter than Mk.I. Basic design was the same as Mark 1, with omission of all wooden parts of Mk.I and shorter barrel jacket, which made the Mk.II lighter than its predecessor. Magazine housing could be rotated for about 90 degrees down to close feed and ejection apertures during transportation and off-battle carry (this feature caused much troubles as the rotary unit was not very durable and magazine could be misaligned during combat, what led to feed malfunctions and jams). Another source of problems was magazine spring, so magazines were routinely loaded with 28-30 rounds instead of "full capacity" 32 rounds to reduce strain on the magazine spring.

Some Mk.II STEN guns were manufactured with integral silencers for undercover operations and were marked as Mk.II(S). These guns had shortened barrels enclosed into integral silencer. The silencer was rather effective so most audible sound when firing Mk.IIS was the clattering of the bolt moving back and forth in the receiver. Contemporary manuals advised that Mk.IIS submachine gun was to be fired in semi-automatic mode; the ful-automatic fire was reserved for emergency situations, as it decreased the service life of silencer significantly.

The STEN Mk.III was modification of Mk.I. The major change was that the receiver and the barrel shroud were made from single tube (wrapped from sheet-steel and welded at the top) that extended almost to the muzzle. Another changes included fixed magazine housing for improved reliability and small finger guard in the front of the ejection port. Internally, Mk.III was similar to Mk.I and has same variety of skeleton stocks. Mk.III first appeared in 1943.

The STEN Mk.IV was made in experimental form only, and did not entered the production. It was originally intended for airborne troops.

The STEN Mk.V submachine gun was an attempt to made Mk.II a more "good looking'" gun. Being internally the same as Mk.II, the "STEN Mk.V machine carbine" featured wooden buttstock and rear pistol grip, new front sight and bayonet mount. Early Mk.V's also featured wooden front grips, but these were prone to breakage and thus were removed soon. STEN Mk.V appeared in 1944 and remained in service until the early 1960s', and then replaced by Sterling submachine guns.

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